1. Field of the Invention
The present invention relates in general to the use of copper in semiconductor devices. More particularly, it relates to a method for capping over a copper layer to reduce copper hillock formation and improve adhesion of a capping layer to an underlying copper layer.
2. Description of the Related Art:
In the fabrication of semiconductor devices, integrated circuits utilize multilevel wiring structures for interconnecting regions within devices and for interconnecting one or more devices within the integrated circuits. Conventionally, formation of such structures provides, first, lower level wiring lines and then a second level wiring line in contact with the first wiring lines.
Aluminum and aluminum alloys are traditional metal interconnect materials. While such materials have been used as metal interconnects, concern exists as to whether aluminum will meet the demands required by circuit density and higher semiconductor device speeds. Because of these concerns, other materials have been researched for use as interconnects in integrated circuits.
Since copper has a lower resistivity and reduced susceptibility to electromigration failure compared to traditional aluminum or aluminum alloys, it is widely applied to multilevel interconnects in semiconductor devices.
A problem with the use of copper as an interconnect material is that it easily diffuses into surrounding dielectric materials, especially silicon dioxide. In order to inhibit the diffusion, copper interconnects are often capped. An insulating layer, such as silicon nitride, is typically used as a capping layer to cap the top surface of copper interconnects. In general, silicon nitride is formed by plasma enhanced chemical vapor deposition (PECVD). However, in using a silicon nitride cap for copper interconnects, conventional PECVD silicon nitride creates reliability problems. For example, silicon nitride films deposited by PECVD have poor adhesion to copper surfaces. Therefore, when subsequent dielectric layers are deposited onto the silicon nitride film, stress produced therein causes the nitride film to peel from the copper surface and creating a path for copper to diffuse outward and moisture or other contaminants to diffuse inward.
In addition, copper is easily oxidized when exposed to air or oxygen ambient lower than 200xc2x0 C., thus copper oxide is formed thereon. When a heat treatment, such as annealing, is performed on the copper for subsequent processes, the copper and the overlying copper oxide expand due to heating, but different thermal expansion coefficients of both induce compressive stress at the interface therebetween. In order to release the stress, copper hillocks form on the copper surface, possibly resulting in shorts between neighboring copper interconnects.
It has been suggested to use a copper silicide film on copper interconnects to improve the interface adhesion of copper to silicon nitride. In U.S. Pat. No. 6,303,505, the adhesion problem of capping a silicon nitride layer to a copper interconnect is addressed by treating the exposed copper surface with hydrogen plasma and then reacting the treated surface with silane or dichlorosilane to form a copper silicide layer thereon. Moreover, in U.S. Pat. No. 6,492,266, adhesion problems are addressed by similar method, but ammonia plasma is used. In addition, in U.S. Pat. No. 6,518,167, a metal or metal nitride layer is formed from reaction between a metal organic gas or metal/metal nitride precursor and the copper layer to serve as an adhesion layer between the copper layer and an overlying silicon nitride capping layer.
However, copper silicide exhibits a relatively high resistivity, increasing the resistance of copper interconnects. In addition, neither the copper silicide layer nor metal nitride layer has a suitable thermal coefficient to the underlying copper layer, causing copper hillock growth or formation after heat treatment.
Accordingly, an object of the present invention is to provide a novel method for capping over a copper layer, which uses two-step plasma treatment to suppress copper hillock formation and enhance adhesion of the copper layer to an overlying capping layer, thereby increasing reliability of the capped copper layer.
According to the object of the invention, a method for capping over a copper layer is provided. First, a copper layer is formed overlying a substrate. Next, a first plasma treatment is performed on a surface of the copper layer. Subsequently, a second plasma treatment is performed on the surface of the copper layer. Finally, the copper layer is capped with an insulating layer.
The first plasma treatment is performed using hydrogen as a reacting gas at about 300 to 500xc2x0 C., for about 5 to 15 seconds, at a pressure of about 3 to 6 Torr.
The second plasma treatment is performed using ammonia as a reacting gas at about 300 to 500xc2x0 C., for about 5 to 20 seconds, at a pressure of about 2 to 4 Torr. Moreover, the reacting gas further comprises nitrogen.
Moreover, the insulating layer can be a silicon nitride (SiN) layer, a silicon carbide (SiC) layer, a silicon carbonitride (SiCN) layer, or a silicon oxycarbide (SiCO) layer.
Another aspect of the invention provides a method for forming a copper interconnect. First, a substrate covered by a dielectric layer is provided. Next, the dielectric layer is etched to form an opening therein. Thereafter, the opening is filled with a copper layer to serve as the copper interconnect. Next, a surface of the copper layer is treated with hydrogen-containing plasma. Subsequently, the surface of the copper layer is treated with nitrogen-containing plasma. Finally, a capping layer is formed on the dielectric layer and the copper layer.
The surface of the copper layer is treated with hydrogen-containing plasma at: a temperature of about 300 to 500xc2x0 C.; a period of time of about 5 seconds to 15 seconds; and at a pressure of about 3 to 6 Torr.
The surface of the copper layer is treated with nitrogen-containing plasma at about 300 to 500xc2x0 C., for about 5 to 20 seconds, at pressure of about 2 to 4 Torr. Moreover, the reacting gas further comprises nitrogen.
Moreover, the capping layer can be a silicon nitride layer, a silicon carbide layer, a silicon carbonitride layer, or a silicon oxycarbide layer.